Communication system employing pulse code modulation



w. M. GOODALL 2,449,467 COMMUNICATION .-SYSTEM EMPLOYING Sept. 14, 1948.

PULSE CODE MODULATION I 12 Sheets-Sheet 3 Filed Sept. 16, 1944 INVENTOR W M GOOD/ML ATTORNEY Sept. 14, 1948.

Filed Sept. 16, 1944 W. M COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION FIG. 4

12 Sheets-Sheet 4 INVENTOR Hz M GOODALL ATTORNEY Sept. 14, 1948. w. M. GOODALL 2,449,467

7 COMMUNICATION SYSTEM EMPLOYIN PULSE CODE MODULATION Filed Sept. 16, 1944 12 Sheets-Sheet 5 FIG. 5'

INK/EN TOR M. M GOOD/ILL A TTO/PNE Y Sept. 14, 1948. w. M. GOODALL 2,449,467

. COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed Sept. 16, 1944 12 Sheets-Sheet 6 INV N Maw 2M.

A T7DRNE Y Sept. 14, 1948. w. M. GOODALL 2,449,467

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed Sept. 16, 1944 12 Sheets-Sheet '7 INVENTOR M! M GOODALL n/Jaw A TTDRNEY w. M. GOODALL COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Sept 14, 1948.

12 Sheets-Sheet 8 Filed Sept.- 16, 1944 lNl/ENTOR M GOODALL ATTORNEY Sept. 14, 1948. w. M. GOODALL 2,449,467

COMMUNICATION SYSTEM EMPLOYING I PULSE CODE MODULATION Filed Sept. 16, 1944 12 Sheets-Sheet 9 INVEN TOR w M GOODALL FIG. 9

A TTORNE Y Sept. 14, 1948. 'w. M. GOODALL COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATIO 12 Sh e ets-Sheet 1o Filed Se t. 16, 1944 7 new q imuk INVEN TOR W M GO0DALL Mum 5M.

ATTORNEY Sept. '14, 1948.

Filed Sept. -16, 1944 Ill-3 FIG ' W. M. GOODALL COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION 12 Sheets-Sheet 11 m/ VENTOR By WMGOQDALL ATmRNEY v Sept. 14, 1948.

Filed Sept. 16, 1944' FIG. [2

. W. M. GOODALL GOIIUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION 12 Sheets-Sheet 12 FIG. /4 as 11404 I405.

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mwzurok w M GOODALL A TTOR NE) CATION SYSTEM EMPLOYING ULSE CODE MODULATION William M. Goodall, Oakhurst, N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 18, 1944, Serial No. 554,495

26 Claims. I

'This invention relates to communication systems for the transmission of complex wave forms of. the type encountered in speech, music, sound, mechanical vibrations and picture transmission by means of code groups of a uniform number of signal impulses of a plurality of difierent types or signaling conditions transmitted at high speed.

The object of the present invention is to provide a communication system capable of transmitting and ,reprodu-clng with high fidelity a complex wave form over an electrical transmission path in such a manner that the signal-to-noise ratio of the received signal is materially improved and at the same time the frequency band width required for the transmission of the signals is reduced to a minimum.

Another object of this invention is to provide improved and simplified methods and apparatus capable of transmitting and receiving signal impulses over a noisy transmission system and deriving therefrom signals having a high signal-tonoise ratio.

In the past, various communication systems 'have been proposed for improving the signal-tonoise ratio of the received signals. In one such system the amplitude of the signal wave to be transmitted is sampled at successive intervals and pulses transmitted having either their beginning or termination or both a function of the amplitude of the complex wave. While such systems may secure an improvement in the signal-tonoise ratio they require either a wide frequency band or a long time interval or both for the transmission of the pulses representing a single amplitude.

Other systems have been proposed in which a group of pulses are transmitted over a plurality of separate or individual transmission paths. Such an arrangement is complicated and cumbersome and requires a plurality of transmission paths for the transmission of each of the complex waves to be transmitted. Furthermore, in most of the systems in the prior art, it is necessary to count .9, large number of pulses in order to determine the character and position of each pulse of each group of pulses to be transmitted representative of a given amplitude.

It is an object of the present invention to provide methods of and circuits and apparatus for transmitting in succession a group of pulses in sequence over a given channel representative of the amplitude of a complex wave at successive instants of time.

It is another object of the present invention to provide improved apparatus for determining 2 the code to be transmitted to represent each of a large number of different amplitudes without the use of complicated counting circuits, arrangements and equipment.

Another object of the present invention is to transform a series of pulses representing the amplitude of a complex wave at a given instant of time into another pulse having an amplitude which is a function of the amplitude of the original complex wave.

Another object of this invention is to recom bine a group of successive pulses of varying amplitude in such a manner as to reconstruct a wave form of substantially the same shape as the wave form transmitted.

A feature of the present invention relates to apparatus and circuits for determining the magnitude of an electrical quantity and transmitting a series of pulses representative of said magni- 'tude.

Another feature of the present invention relates to apparatus and equipment for producing a wave form in the form of a plurality of steps of increasing or decreasing amplitude, the height or change in amplitude between successive steps being a predetermined fraction of the change in amplitude between the immediately preceding steps.

A feature of this invention resides in method of and apparatus for varying the charge on a condenser by successively smaller amounts when the pulses of a code are received.

Still another feature of this invention relates to methods of and apparatus for varying the amplitudes of each of the pulses of a received pulse group so that the amplitude of each pulse is less than the amplitude of the previous pulse of the code group.

Another feature of the invention relates to a sampling apparatus for sampling a complex wave at frequent intervals of time.

In accordance with another feature of this invention an electrical quantity is obtained which is proportional to the amplitude of the complex wave at each time the complex wave is sampled.

Still another feature of this invention rel-ates to apparatus for comparing said electrical quantity with the above-mentioned step-wave form and transmitting pulses under controlof said apparams for comparing said quantity and said stepwave form.

Another feature of this invention comprises equipment responsive to each impulse transmitted ior causing a reduction in the magnitude of said amass? Other features of the invention relate to synchronizing and coordinating the various circuits and equipment at the transmitting terminal with each other and with the circuits and equipmentof the receiving end so as to secure proper operation of the entire system.

A feature of the synchronizing and coordinating circuits and equipment relates to means for accurately pulsing, starting, or phasing an oscillator circuit for a timing circuit for each code or number group of the pulses transmitted.

Briefly, in accordance with the present invention, equipment is provided responsive to a synchronizing or controlling alternating current for generating a control pulse or a group of control pulses of predetermined time relation one with another. These control pulses are employed to control a code element timing circuit which circuit in turn generates a series of very short positive pulses interspersed with a series of very short negative pulses. The control pulses as well as the code element timing'pulses are employed to generate a stepped-wave form of a plurality of steps, the change between each succeeding step of which is a predetermined fraction of the change between the previous steps.

Apparatus is also provided for sampling or deriving an electrical quantity which is a function of the'amplitude of the complex wave to be transmitted under control of a control pulse generator. For each of the control pulses, a code element timing circuit generates a series of code element timing pulses, the step generator circuit generates a wave form having a plurality of steps as described above and the sampling circuit derives an electrical quantity having a magnitude related to the magnitude of the complex wave at the time of the control pulse. The magnitude of the electrical quantity is then compared with the magnitude of the stepped-wave form and if the electrical quantity exceeds the magnitude of the particular step at that time a pulse is transmitted over the transmission system. If the magnitude of the stepped-wave form exceeds the magnitude of the electrical quantity, no pulse is transmitted. Upon the transmission of a pulse the electrical quantity is reduced in magnitude. Thereafter the next step of the stepped-wave form is prepared for comparison with the remaining magnitude of the electrical quantity and the process repeated a predetermined number of times and the corresponding pulses transmitted. The amount subtracted from the electrical quantity as an incident to the transmission of each pulse is the total maximum value of the quantity multiplied by l/A" where A is a constant greater than 1 and usually 2 and n is the ordinal or digital number of the pulse transmitted. At the receiving station a control pulse generator, code element timing generator and a stepped-wave form generator similar to those of the transmitting station are also provided. In addition, decoding apparatus is provided in which the received pulses are employed to synthesize an electrical quantity having a magnitude similar to the magnitude of the electrical quantity of the transmission end of the system and then the complex wave is reconstructed from said electrical quantity.

Novel features of this invention which are believed to be characteristic thereof are set forth with particularity in the claims appended hereto. The invention itself, however, both as to its organization and method of operation together with other objects and features thereof may be best understood from the following description when read with reference to the accompanying drawings in which: Figs. 1 and 2 show in diagrammatic form. the

various elements and the manner in which they cooperate to form an exemplary communication system embodying the present invention;

Figs. 3 to 10 when positioned as shown in Fig. 16 show in detail the various circuits and equipment of an exemplary communication system embodying the present invention; of these Figs. 3 and 5 may be common to all channels at a transmitting terminal, and 8 and 9 are common to all channels at a receiving terminal;

Figs. 11 and 12 show curves or wave forms of the voltages or currents at various places in the system at the transmitting end or station;

Figs. 13 and 14 show similar curves and wave forms at the receiving station;

Fig. 15 shows the manner in which Figs. 1 and 2 are placed adjacent each other;

Fig. 16 shows the manner in which Figs. 3 through 10 should be placed adjacent each other; and

Fig. 17 shows the manner in which Figs. 11

through 14 should be placed adjacent each other.

Figs. 1 and 2 show in schematic form the various circuit elements and apparatus" and the manner in which they cooperate to form an exemplary system embodying the present invention. Fig. 1 shows the apparatus and circuits at one station 'while Fig. 2 shows similarapparatus and circuits at a second station. As shown in Figs. 1 and 2, two transmission paths or channels are provided in each direction.

It will be obvious to persons skilled in the art that additional channels in either or both directions may be provided between each of these stations. It will also be obvious to persons skilled in the art that additional stations or terminals may be provided and similar transmission paths or channels extended between any or all of the various stations.

As shown in Figs. 1 and 2, lines 8 and 9 represent two communication paths between the two stations. Signals are transmitted over line 6 from the station shown in Fig. 1 to the station shown in Fig. 2. Signals are transmitted over line 9 from the station shownin Fig. 2 to the station shown in Fig. 1. These lines 6 and 9 represent communication paths between the two stations which may be of any suitable type for the transmission or conveyance of electrical signaling pulses of the proper frequency range. These paths may be used independently of each other or in pairs to Provide two-way circuits. These paths are illustrated in Figs. 1 and'2 by a single line. Persons skilled in the art, however, will readily understand that these lines represent any and all types of communication paths and combinations thereof including open wire lines, cable conductors, twisted pairs, channels of carrier current systems, coaxial lines, wave guides, and radio channels and may also include time division multiplex channels. Furthermore these transmission paths may include suitable amplifiers, filters, -.predistortion networks, equalizing networks, gain control networks and circuits, phase control circuits and networks, repeaters, repeater stations, as well as signal operated switchingdevices which control the direction of transmission. In addition, when the transmission path is over a coaxial cable the transmission system radio channel between these stations.

may include any or all the features disclosed in or referred to in United States Patents 2,343,568 granted to L. W. Morrison, Jr., on March 7, 1944, 2,212,240 granted August 20, 1940, to Lalande et al.; and 2,095,360 granted October 12, 1937, to Green. These features may also be employed for other types of conductors when desirable. These channels may also include regenerative repeaters as well as the other terminal equipment associated with each of the respective types of transmission paths suitable for terminating and interconnecting the various types of transmission lines so that they will each cooperate with the adjacent sections to form a continuous transmission channel between the two stations. In addition to lines 6 and 9, two radio channels are shown in Figs. 1 and 2. Transmitting antenna I and receiving antenna 8 represent one of .the radio channels, while wave guide I3 and horn or radiator I2 and the horn II and wave guide I4 illustrates. second A typical example of a radio channel including radio relay repeat-er stations suitable for the transmission of pulses of the type employed in the exemplary system described herein is described in detail in a series of papers published in the Proceedings of the Institute of Radio Engineers for November 1934 entitled An experimental television system," part 1 of which is entitled Introduction which is by E. W. Engstrom; part 2 of which is entitled The transmitter and is by Kell, Bedford and Trainer; part 3 of which is entitled The receivers and is by Holmes, Carlson and Tolson; and part 4 which is entitled The radio relay link for television signals and is by Young. Another sutable type of radio amplifier repeater and transmitter is described in detail in an article entitled High gain amplifier for 150 megacycles" by Rodwin and Klenk published in the Proceedings of th Institute of Radio Engineers for June 1940. Typical wave guide and horn or radiator structures are described in United State Patent 2,206,923 granted to Southworth July 9, 1940. The foregoing publications and patent are hereby made a part of the present applicationto the same extent as though set forth in full herein.

In addition a synchronizing path or channel 5 is shown extending between the two stations shown in Figs. 1 and 2. This control path or channel may be similar to the other transmission paths between the stations. Furthermore, if it is so desired the synchronizing signals or the control frequency may be transmitted over one more or more of the other transmission paths extending between the two stations. Inasmuch as there are numerous types of synchronizing apparatus in the art which will operate over the same transmission paths as employed for the transmission of communication signals and since the operation of this type of equipment is well-known and understood by persons skilled in the art, it is considered unnecessary to further expand the present disclosure to show details of a typical system of this type. It is understood, or course, that such equipment will cooperate with the various circuits of the present invention and may be provided when it is so desired.

. terminate at each or the respective stations.

The common equipment at station I comprises a control oscillator I I0 which may be of any suitable type, as for example, the types described in detail in any one or more of the following patents: 1,476,721, Martin, December 11, 1923; 1,660,389, Matte, February 28, 1928; 1,684,455, Nyquist, Septggagber 18, 1928, and 1,740,491, Afiel, December 24, 1

The output of the control oscillator is coupled to control a control pulse generator I I I. The output of this generator extends to receiving and monitoring equipment as shown in the drawing and also to delay network I I2. Delay network I I 2 may be any suitable type of delay network as, for example, one or more sections of one or more of the types disclosed in United States Patent 1,770,422, granted July 15, 1930, to Nyquist. The output of delay network II2 extends to sampling circuits I22 and I32 and also-to a code element timing circuit H3. The output of the code element timing circuit extends to the coding and comparing circuits I23 and I33 and also to a stepwave generator I I4. The output of the step-wave generator extends to the sampling circuits I22 and I32 and also to the coding circuits I23 and I33.

Similar common equipment comprising a control oscillator 2I0, control pulse generator 2, delay network 2| 2, code element timing circuit 2I3 and a step-wave generator 2 are provided at the station shown in Fig. 2.

In addition to the control oscillator H0 and 2 I0 respectively at each of the control stations a master oscillator I0 is shown in Fig. 1. This master oscillator may be located at either of the stations I or 2 and when so located at either of these stations, may replace the control oscillator III! or 2I0 at either of these stations. However, the master oscillator is frequently located at some central point and provides a control frequency for an entire nationwide system or for some smaller divisions or sections of a large sys tem. Typical oscillators and standard frequency systems suitable for use as a master oscillator or source of control frequency are disclosed in United States Patents 1,788,533, Marrison, January 13, 1931; 1,931,873, Marrison, October 24, 1933; 2,087,326, Marrison, July 20, 1937; 2,163,403, Meacham, June 20, 1939; and 2,275,452, Meacham, March 10, 1942.

All of the patents referred to above are hereby made a part of the present application as if fully included herein.

The sources of signals I29 and I30 in Fig. 1

and 220 and 230 in Fig. 2 are illustrated in the drawing by microphones. It will be apparent to persons skilled in the art that other suitable sources of signals may be employed such as picture transmission systems, television systems, mechanical vibration pick-ups, photoelectric devices, piezoelectric devices, etc. 4

Each of these sources is connected to the respective terminal equipments I2I, I3I, HI and 23I. This terminal equipment represents all of the equipment connected between the sources or microphone and transmission circuits and equipment for coding the signals to be transmitted as will be described hereinafter. This terminal equipment may include one or more of the following typical types of apparatus including amplifiers, transmission lines, switching equipment of any suitable type such as manually controlled switching equipment at a manual central oflice, automatic or machine or dial switching equipment such as employed in an automatic central office as well as phase control apparatus, equalizing apparatus; voice controlled switching apparatus, etc. This terminal equipment may also include toll line facilities and toll line switching equipment. When the terminal equipment I2I, I3l, 22I' or 23I includes switching equipment the transmission circuits described hereinafter in detail embodying the present invention function as trunk circuits.

The signals from the devices I20, I30, 220 and 230 then extend to sampling circuits I22, I32, 222 and 232 respectively. -Associated with each of the samplin circuits is a comparing or coding circuit I23, I33, 223 and 233. From the comparing coding circuit the signals are transmitted through the associated high frequency transmitters to the other station. After the signals have been received at these stations they are transmitted through a phase or time delay circuit H4, I53, 244 or 253 which may be constructed of a suitable number of network sections of a suitable frequency range of one or more of the types of the above-identified Patent 1,770,422 granted to Nyquist. As will be readily understood by persons skilled in the art, it is.

necessary that the incoming signals be maintained in a predetermined phase relation with the common equipment at the receiving stations so as to be properly decoded and translated so that the original complex wave may be reconstructed. It is, of course, possible to provide the phase or delay equipment at each of the stations for each of the transmitting and receiving terminals located thereat. It is also possible to provide such equipment in the synchronizing channel. However, as shown in Figs. 1 and 2 the phase control equipment has been provided at the receiving terminals of the channels terminating at the respective stations. This arrangement has the advantage that the phase control equipment may be adjusted to compensate for the delayed time of each of the respective channels so that it is possible to employ common control equipment at each of the stations for the operation of each of the channels terminating thereat. After passing through the receiving and phase control equipment at the respective stations, the signals are decoded by the respective decoding circuits I42, I52, 242 and 252 and conveyed to the respective terminal equipment I4I, I5I, 2M and 25! and then to the receiving or recording devices I40, I50, 240, or 250, respectively. The terminal equipment MI and ISI, 2 and 25I may include any of the types of apparatus or equipment described abovewithreference to the terminal equipment I2I, I3I, 22I or 23I.

Inasmuch as the circuits individual to each of the channels all operate in substantially the same manner'the circuits of a single channel only have been shown and described herein in detail. It must be understood, however, that similar circuits are duplicated for each of the additional channels extending between any of the stations of the system.

A control pulse generator III is provided at the transmitting station and connected to the output of the control oscillator III] or the master oscillator I0. The control pulse generator III is arranged to generate control pulses of both positive and negative polarity in response to the alternating current or voltage applied thereto from the control oscillator III) or the master ossistances, inductances, and capicities as pointed out above. The output of the delay network H2 is connected to a code element timing circuit I I3, 9. step generator H4 and a sampling circuit I22 In response to each of the control pulses applied to the code element generator or timing circuit H3 a series of code element timing pulses is generated. Each series of code element timing pulses comprises a group of positive pulses interspersed with an equal number of negative pulses. A positive pulse and a negative pulse are generated by the code element timing circuit for each pulse of each code group of the signalling pulses as will appear hereinafter. The output of the code element timing circuit is connected to the step generator H4 and also to comparing and coding circuits I23 and I33. Step generator H4 is employed to generate an output wave in the form of a plurality of steps, the height of each succeeding step being a predetermined fraction of the height of the previous step. In the exemplary embodiment of the presentinvention theheight of each succeeding step is one-half the height of the previous step. Step generator II4 generates a step for each of the negative pulses applied thereto by the code element timing circuit H3.

In addition the entire step wave pattern from the step generator H4 is generated for each of the control pulses applied thereto from the control pulse generator III through the delay network I I2. The output of the step generator I I4 is connected to sampling circuits I22 and I32, to coding and comparing circuits I 23 and I33, to the decoding circuits I42 and I52, and to monitoring equipment I25, I35, I45 and I55. The outputs of the comparing and coding circiuts I23 and I33 are connected to transmitting amplifier I24 or radio transmitter I34 as well as to the monitoring equipment I25 and I35 and the sampling circuits I22 and I32.

Briefly the operation of the system is as follows. Assume for example that the input speech, music or picture signal from microphone or other device I20 has a wave form such as illustrated by curve IIIII of Fig. 11. Assume further that the frequency of the master oscillator I0 is such that a control pulse is generated by the control pulse generator II I at the times corresponding to each of the large dots '02 on curve IIDI. At each of these times a control pulse is transmitted through the delay network II2 to the sampling circuit I22 where some electrical quantity such as, the charge on a condenser is made proportional to the instantaneous amplitude of the complex wave at that time. In addition, the code element timing circuit and the step generator are set into operation. The outputs of the sampling circuit and the step generator are compared in the comparing and coding circuits I23. If the sample obtained from the complex wave as described above is greater than the amplitude of the step wave from stepgenerator H4 at the time of the first positive pulse from the code element timing circuit 3, a signalling pulse is transmitted from the comparing and coding circuit I23. If

the amplitude of the wave from the step-wave generator is greater than the amplitude of the sample at this time, i. e., is greater than the charge of the condenser referred to above, no signalling pulse will be transmitted at this time. Assuming for purposes oi! illustration that the charge or potential on the condenser in thesampling circuit is greater than the amplitude of the step wave when the first positive pulse is transmitted from the code element timing circuit, then the pulse will be transmitted from the comparing and coding circuit I23. This pulse is transmitted through the transmitting amplifier I24 over line 6 to the receiving station. This pulse is also transmitted to the monitoring equipment I25 and also to the sampling circuit I22. When this pulse arrives at the sampling circuit I22 it has been delayed somewhat bythe amplifiers and if necessary other delay equipment, so that it will not interfere with the pulse actually transmitted from amplifier I24 over line 6. When this transmitted pulse arrives at the sampling circuit I22, it causes the charge or potential of the condenser therein to be reduced by an amount controlled by the amplitude of the step wave generated by the step wave generator I I4. As assumed above, when the step wave will have its first amplitude, the charge or potential of the condenser in the sampling circuit will be reduced by an amount equivalent to substantially half of the maximum charge which may be applied to that condenser by a signal wave having its maximum possible amplitude at the time of sampling.

Thereafter the code element timing circuit I13 will transmit a negative pulse to the step generator which will cause the output wave form to assume its next value. This value will then be compared with the remaining charge or potential on the condenser of the sampling circuit and another pulse transmitted or not, depending upon whether or not the charge on the condenser in the sampling circuit is greater or less than the magnitude of the second step from the step generator. Assuming for purposes of illustration that the potential of the second step of the step generator is greater than the charge on the condenser in sampling circuit I23, no pulse would be transmitted. Consequently, the charge upon the condenser in the sampling circuit will not be reduced.

A short interval of time later another negative pulse will be transmitted from the code element timing circuit I I3 and cause the output of the step generator II4 to assume the value of the next step. This is again compared with the potential on the condenser in the sampling circuit in order to control the transmission of the next pulse. The above-described cycle of operation is then repeated a predetermined number of times, depending upon the number of code elements employed to represent each of the amplitudes sampled by the sampling circuit. In an exemplary embodiment of th invention described herein it has been assumed that six code pulses will be sufiicient to represent the various amplitudes of the complex wave.

With the six code pulses it is possible to represent 64 difierent magnitudes of the instantaneous amplitude of the complex wave. If five code pulses are employed, 32 different amplitudes may be represented by the difierent code combinations. If it is desired to more accurately determine and represent the instantaneous amplitude of the complex wave form more code pulses may be employed. For example, if seven code pulses are employed 128 different amplitudes may be represented. In other words 2 different amplitudes may be represented by the permutations oi n pulses oi. two different types, 1. e., current or no current. It will be readily understood by persons skilled in the art that in order to change the number 0! code elements representing each amplitude it is only necessary to change the number of pulses generated by the code element timing circuit 3 in response to each of the control pulses transmitted through the delay network II2.

After all of the pulses of a given code group are transmitted another control pulse from the control pulse generator III will cause the equipment to sample the complex wave form at a succeeding interval of time and the above process is repeated. The interval of time between samples of the complex wave, together with the number of different amplitudes representing each of the instantaneous amplitudes determine the accuracy and fidelity of the reproduction of the complex wave form at the receiving station as will be readily apparent to persons skilled in the art.

Monitoring equipment I25 is provided at the transmitting station and is connected to the control pulse generator III, step generator H4, and the output of the comparing and coding circuit I23 and is arranged to decode each group of code pulses and generate a pulse having an amplitude proportional to the instantaneous amplitude represented by the code group of pulses transmitted. Each of these pulses of varying amplitude generated in the monitoring circuit is applied to a low pass filter and then to some receiving device or indicator I26 so that the operation of the system may be monitored at the transmitting station. A monitoring circuit is shown in Fig. 1 for each channel. It will be apparent to persons skilled in the art that the monitor'circuit may and usually will be common to a number of the channels and connected to any desired channel by keys, a. plug and jack or other suitable equipment.

The station shown in Fig. 2 is similarly provided with a control pulse generator 2I I which generates a pulse in response to each oscillation received over channel 5 and from oscillator 2-I0. However, in the event that channel 5 becomes inoperative, the control pulse generator 2 will still be controlled by oscillator 2I0. Consequently, it may be possible to operate the system for a considerable period of time. without receiving a synchronizing or controlling frequency over channel 5. Y

The receiving station is likewise provided with a delay network -2l2, code element timing circuit 2I3, and step-wave generator 2I4 which operates in substantially the same manner as described above with reference to a transmitting station. The output of the step generator 2 is connected to the decodingcircuit 242 to which is also applied the received signals after being amplified by amplifier 243. The decoding circuit 242 changes the constant amplitude of each of the received pulse groups into pulses of varying amplitude depending upon the amplitude of the step wave at the time the individual pulses of the group are received. The pulses of varying amplitude are then applied to a low pass filter where the complex wave form is thereupon reconstructed or regenerated. The output of this filter is then connected to terminal equipment 24I where it is transmitted to the receiving device 240. Terminal equipment 24I may include any or all of the equipment mentioned above with reference to terminal equipment |2|. It will be readily understood by persons skilled in the art that terminal equipment |2| and 2 may each include any of the different types of equipment referred to above but that both of these devices do not necessarily have to include the same types of equipment but may include such equipment.

Monitoring equipment 245 is provided at the receiving station and enables the attendants to check the operation of the receiving apparatus. This equipment as shown in Fig. 2 is individual to the transmission path 8. It may be common to all the paths terminating at the station of Fig. 2 in which case it would be switched to the transmitting or receiving end of any path over which it was desired to observe the transmission.

. Reference will now be made to Figs. 3 through 10 inclusive when arranged as shown in Fig. 16 which set forth in detail the various circuits and apparatus which cooperate to form a typical exemplary communication system embodying the present invention.

Figs. 3 through '7 inclusive illustrate in detail the common equipment employed at one station at which it is assumed the complex wave form is applied to the system. This station will irequently be called a transmitting station hereinafter. Figs. 8, 9 and 10 show the equipment at a second station at which the complex wave form is reconstructed and delivered to utilization circuits and equipment. This station will be frequently called hereinafter a receiving station. In addition. Figs. 3 through 10 inclusive show only a single one way communication path and the equipment thereof in addition to the equipment common to a plurality of paths. Figs. 3 and 5 show details of circuits at the transmitting station which are common to a number of transmission paths and Figs. 8 and 9 show similar circuits at the receiving station. The other figures show the equipment which is individual to each transmission path. It will be readily appreciated by persons skilled in the art that to provide additional communication paths in either direction between two stations, it is only necessary to provide additional equipment similar to the equipment shown in Figs. 4, 6, 7 and which is indivdual to each of the transmission paths.

In order to better understand the operation of the system the common equipment shown on the top of Figs. 1 and 2 in diagrammatic form will be described first.

Fig. 5 illustrates a master oscillator 5||I and the second oscillator 525. If the master oscillator 5||l is located at the transmitting station the details of which are illustrated in Figs. 3, 4, 5, 6 and 7 the local oscillator 525 may be dispensed with. However, in case the master oscillator 5H! is located at some other station or is a master frequency standard for a large number of stations, systems, or for the entire country, both oscillator 5H! and the local oscillator 525 will usually be employed. Master oscillator 5H] may be of any suitable type such as the type disclosed in the above-identified Meacham, Marrison or Morrison patents. The local oscillator 525 will then incorporate some control apparatus for maintaining its frequency in synchronism with the frequency from the master oscillator -5|0 similar to the equipment described in detail in the above-identifled patents. Oscillator 525 is connected over a synchronizing line 580 which is shown in Fig. 5 as a coaxial line and extends to the receiving station shown in Figs. 8, 9 and 10. Here the coaxial prising tube III.

line 580 terminates in a local oscillator 8|0 which is similar to the oscillator 525. While the synchronizing line 580 is shown as a coaxial line, persons skilled in the art will readily understand that any suitable type of transmission path may be employed which is capable of transmitting the synchronizing frequency employed.

Pulse generator The local oscillator 525 or the master oscillator III! is connected to a multi-vibrator circuit com- The multi-vibrator circuit 5| 1 operates to generate square waves of the same frequency as received from oscillator 525 or 510. Multi-vibrator circuits are well known in the art. Typical multi-vlbrator circuits suitable for use in the present system are described in United States Patents 1,744,935 granted to Van der Pol January 28, 1930, and 2,022,969 granted to Meacham on December 3, 1935, and in an article by Hull and Clapp published in the Proceedings of the Institute of Radio Engineers for February 1929, pages 252 to 271. See also section 4-9 Multivibrator" on page 182 of Ultra-High-Frequency Techniques by Brainerd, Kochler, Reich and Woodruil. The output of the multi-vibrator 5|0 is coupled through a condenser 5|2 and a resistance 5|! to amplifier tube 5.

Condenser 5|2 is made variable so that it, together with resistance 5|3 may be employed to control the length of the synchronizing pulses derived from multi-vibrator circuit 5| If the time constant of condenser H2 and resistance 5|3 is large the output pulse will be relatively long, whereas if the time constant of condenser 5|! and resistance 5|3 is small the output pulse will be short. In a typical example of the present system the values of condenser 5|2 and resistance III were selected to produce an output of approximately two microseconds.

Condenser 5|2 and resistance 5|3 are coupled to the control grid of amplifier 5. The output of the amplifier tube 5 is in turn coupled to tubes III and 5|6. Tubes 514, 5|5 and 5|-6 are amplifier tubes which are overloaded by the magnitude of the pulse applied to them so that these tubes tend to limit the magnitude of the pulse repeated through them and at the same time tend to make it square in wave shape. Amplifiers of this type are sometimes called "1imiters and at other times "clipping amplifiers because they limit, clip of! or suppress the top portion of the waves applied to them. Asingle stage limiter is shown in Fig. 8-6 on page 282 and described on page 283 of Ultra-High Frequency Techniques by Brainerd, Kochler, Reich, and Woodrufl, first published July 1942 by D. Van Nostrand Company, Incorporated.

The output of tube 5| 6 is coupled to a power tube 5" which is employed to supply sumcient power for the output pulses of the circuit so that they may be employed to control the other circuits of the system. The output tube 5|1 is arranged to supply both positive and negative Pulses. vNegative pulses are obtained from the plate of tube 5", while positive pulses are obtained from its cathode as shown in Fig. 5.

. As will be readily appreciated by persons skilled in the art, in case a large number of circuits are supplied from pulse generator shown in Fig. 5, additional output stages may be connected in parallel with tube 5", may have their input circuits connected in parallel with the input circuit of tube 5", or may be driven from this tube as is well understood and frequently employed.

The negative pulses from the plate of tube pass through a delay network 522 where they are delayed slightly in time with respect to positive pulses 521. The purpose of this delay will be explained hereinafter. Delay network 522 will be of any suitable type employing reactive elements in a manner well understood in the art and pointed out above. The output of the pulse generator shown in Fig. 5 is diagrammatically indicated by the small curve 52! for the positive pulses and 523 for the negative pulses in order to facilitate the reading and understanding of the drawing.

In order to further facilitate the understanding of the operation system reference will also be made to Figs. 11, 12, 13 and 14 when arranged as shown in Fig. 1'7. Referring now to Fig. 11, curve H0! represents a complex wave form of a type suitable for transmission over the system described herein. As will 'be described hereinafter the magnitude or amplitude of this wave form is sampled or measured at frequently recurring intervals of time. For purposes of illustration the operation of the system will be described in detail for one such measurement which is assumed to be made at the point I I03.

Vertical lines I I05 in the next set of curves represent the positive pulses 52l supplied by the pulse generator shown in Fig. 5. Likewise, the vertical lines HM represent the negative pulses generated by the pulse generator shown in Fig. 5. As shown in Fig. 11 pulses l I04 are delayed slightly and consequently follow pulses H05. This is apparent in Fig. 11 when it is assumed that time increases in a positive direction to the right.

It is further assumed in the exemplary system described in detail herein that the samples of the complex waves are obtained at a rate of 8,000 times a second. Persons skilled in the art will understand that any other suitable frequency may be employed so long as this frequency is appreciably higher than the highest frequency component of the complex wave necessary or desirable to transmit to the distant station. In other words, the frequency of oscillators 5l0 and 525 are either 8,000 cycles a second or else frequency changing apparatus is connected between them and the multi-vibrator 5| I in a manner understood in the art so that multi-vibrator 5 is supplied with an alternating current of 8,000 cycles per second. In other words, pulses H05 as well as the slightly delayed pulses H04 are supplied from the pulse generator in Fig. 5 at the rate of 8,000 per second. Consequently, there are 125 mircoseconds between pulses H05. Similarly a period of 125 microseconds elapses between the pulses '04.

The negative pulses are supplied to an amplifier tube 3 l 0 shown in Fig. 3 where they are amplified and changed into the positive pulses due to the inversion of the arnplifier tube 3|0. Tube M0 is shown as a double triode tube. However, it is noted that both sections are connected in parallel so that this tube merely serves as a single triode tube. As will be readily understood by persons skilled in the art, this tube may be replaced by any suitable triode tube having a desired output current or power capability. This amplifier also may comprise any multielement tube operated either as a triode with various of the elements connected together or in any other suitable manner.

Code element timing circuit The output of tube 310 is connected to code element timing circuit comprising tubes 3! I, 319, 320 and 32!. The left-hand section of tube 3 is employed to shock-excite the resonant circuit comprising. condenser M2 and inductance ll; connected in parallel in the cathode circuit of the left-hand section tube 3| l. The bias conditions applied to the left-hand section of tube 3 are such that the tube is blocked or non-conducting at all times except when the positive pulse from tube 8"] is applied to its grid. At these times the left-hand section of tube-3H forms a low impedance path for supplying current and energy to the oscillating circuit connected in its cathode circuit. At all other times the anode-cathode circuit of tube 8 is of such a high impedance that it does not materially affect the oscillations of the resonant circuit comprising elements 3". and 3H. The application of a positive pulse to the grid of tube 8 thus sets the resonant circuit described above into oscillation. The wave form of such oscillations is shown by curve 08 in Fi 11.

As shown by curve "08 one suitable type of adjustment for the resonant circuit is such that substantially six complete oscillations take place between the positive synchronizing pulses applied to the grid of the left-hand section of tube 3| I.

In other words, in each period between the synchronizing pulses there is generated one cycle or oscillation for each code pulse of the group of code pulses. If only flve pulses are required to represent the various amplitudes of each sample of the complex wave then the tuning of the resonant circuit comprising condenser M2 and inductance would be varied to generate five cycles or oscillations between synchronizing pulses.

As shown by curve 06 slightly more than six complete oscillations of the resonant circuit take place but the synchronizing pulse causes the circuit to start oscillating from substantially the same point and with the same phase each time it is received. By supplying energy to the oscillating circuit when the current through the coil is small and by utilizing the low impedance of the cathode circuit, transients are maintained small and quickly damped out. Transients do not, therefore, materially affect the frequency or amplitude of the oscillations and at the same time the oscillations are maintained in proper phase.

The cathode of the left-hand section of tube 3| l is connected to the grid of the right-hand section of this tube. The output impedance of the right-hand section comprises a cathode resistor 3 which is of such a value that the right-hand section of tube 3 acts as a so-called cathode follower and thus presents an extremely high impedance to the resonant circuit comprising elements 312 and 3| 3. Consequently, the operation of the right-hand section of tube 3| I does not materially alter or interfere with the operation of the resonant circuit. Such properties and operation of cathode followers are well-known to persons skilled in the art. (See "The cathode follower" by C. E. Lockhart, parts I, II and III, published in Electronic Engineering, December 1942, February 1943, and June 1943, respectively.)

The cathode of right-hand section of tube 3 is coupled through a resistance and capacity network to the grid of tube 3l9. Capacity 3l6 and resistance 315 are employed in the coupling circuit in order to properly control the wave shape of the pulses transmitted to and repeated by the tube 3l9. Resistances 8M, 3l5 and 335 together with the position of potentiometer 3! control or determine the bias of the grid of tube 3| 9. Condenser 3H5 is connected across resistance M5 to compensate for the effect of the input ca- .greater or lesser extent.

pacitance of tube 3I9, thus causing the potential of the grid to rise substantially as fast as the applied potential, i. e.. the cathode potential of the right-hand section of tube 3. The optimum value of condenser 3IIl is the value of the input capacitance of tube 3I3 multiplied by the ratio of resistance 3|! to resistance 335. It should be noted that the potentiometer 3 I3 is connected between the negative source of bias voltage and ground.

The output of tube 3I9 is similarly connected to tube 320 and the output of this tube in turn, connected to tube 32I. Tubes 3I9 and 320 are adjusted to operate as overloaded amplifiers so that they will limit the amplitude of the output pulse and at the same time cause these pulses to approach a square wave form. Tube 32I is a power tube for supplying sufficient output power to operate the other circuits as will be described hereinafter. In this case as in the case of the output of the pulse generator. sufllcient additional output tubes may be provided parallel with or supplied by tube 32I to provide the necessary output currents and voltages as well as to isolate the various diflerent circuits one from another, as may be required.

The amplifier tubes 3I3, 320 and 32I have their circuits and bias potentials so adjusted that a wave form approaching that illustrated by curve or broken line I I01 appears in the output from the tube 32I. Both the positive and negative portions of this wave form as shown in the drawing are substantially o! the same duration. Persons skilled in the art will at once realize that it is not necessary that both of these portions of the wave be of equal or substantially equal duration but may and usually will be of difierent duration to secure optimum operation. Furthermore, these waves are shown to be rectangular in form, as are other waves in the drawing. In practice, the waves are rounded to a Inasmuch as typical actual wave forms approach the wave forms shown in the drawing and would not further aid in an appreciable manner the understanding of this invention the actual waves are represented by the forms shown in the drawing which are much easier to draw and adequately represent the operation of the system. The output of tube 32I passes through condenser'332 to grid of the right-hand section tube 321 and also through condenser 'I2I to the grid of tube "5. The time constants of condenser 332 and resistance 334 and of condenser I2I and resistance I25 is very short so that these condensers act to differentiate the output wave form Hill and apply very short pulses as illustrated in Fig. 11 by H08 and II 09 to the grids of these tubes. The function of these pulses when applied to grids of the respective tubes will be described hereinafter. From an examination of Fig. 11 it is apparent that six positive pulses I I08 and six negative pulses I I 09 interspersed between the positive pulses are generated by the pulse generating circuit for each synchronizing pulse Il04.applied to the code element timing circuit. In other words, a negative pulse and a positive .pulse is generated by the code element timing circuit for each complete oscillation of the resonant circuit comprising condenser 3I2 and inductance 3I3. As will be explained hereinafter the negative pulses 09 are employed to control certain other tube circuits necessary for the proper operation of the system while the positive pulses H08 are employed to cause a transmission of the codesignaling pulses.

Step wave generator potential of the upper terminal of this condenser is reduced to a minimum value. It is not essential that the upper terminal of this condenser be reduced to ground potential at this time, the only requirement being that it be reduced to substantially the same predetermined potential each time that the synchronizing pulse is applied to the grid of the left-hand section of tube 323. When no synchronizing pulse is applied to the grid of this tube the grid is biased to such a value that the tube is blocked; i. e. the anode circuit is substantially cut off. Consequently, this tube has a high impedance at such times and does not materially alter or control the potential of the upper terminal of condenser 330.

When the synchronizing potential is applied to the grid of the right-hand section of tube 323 it likewise causes this tube to become conducting so that condenser 325 in the cathode circuit receives a relatively high positive charge. In other words, the cathode of the right-hand section of tube 323 attempts to follow the potential of the grid of this section and thus the upper terminal of condenser 325 is charged to a relatively high positive potential during the time the synchronizing pulse is applied to the grid of this tube. After the synchronizing pulse is over and until the next pulse arrives the grid bias of the right-hand section of tube 323 is of such a value that the tube is substantially cut oif. Consequently, the right-hand section of this tube does not in any way affect, alter or control the potential of the upper terminal of condenser 325 at these times. The only time in which the righthand section of tube 323 affects the charge on condenser 325 is during the time the synchronizing pulse is applied, as is described above.

After each synchronizing pulse, condenser 325 starts to discharge through resistance 324 connected in parallel with it. The discharge of condenser 325 through the resistance 324 is in accordance with the exponential curve as is well understood by persons skilled in the art. In other words, when the value of resistance 324 and the value of condenser 325 are both fixed as is the usual case, the curve of the potential of the upper terminal of condenser 325 against time follows a well-known exponential discharge curve. See section 123 beginning on page 453 of Principles of Electrical Engineering by Timbie and Bush, published by John Wiley and Sons, Inc., v

1923 (First edition) Although many suitable values for the relative,

magnitudes of condenser 325 and resistance 324 could be chosen to provide many different time constants suitable for operation in circuits described herein, in the exemplary embodiment employed in the present invention a suitable time constant for this network comprising condensers 325 and resistance 324 is chosen so that the potential on the upper terminal of condenser 325 will decrease to one-half its value during each of the pulse intervals. By reference to pulses H09 and H04 it is apparent that one of the pulses H09 is generated a short interval of time after synchronizing pulse I I04 is received. There- 

